﻿using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace System.Linq.Expressions
{
    //public static class PredicateBuilder
    //{
    //    public static Expression<Func<T, bool>> True<T>() { return f => true; }
    //    public static Expression<Func<T, bool>> False<T>() { return f => false; }

    //    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1,
    //                                                        Expression<Func<T, bool>> expr2)
    //    {
    //        var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
    //        return Expression.Lambda<Func<T, bool>>
    //              (Expression.OrElse(expr1.Body, invokedExpr), expr1.Parameters);
    //    }

    //    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1,
    //                                                         Expression<Func<T, bool>> expr2)
    //    {            
    //        var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
    //        return Expression.Lambda<Func<T, bool>>
    //              (Expression.And(expr1.Body, invokedExpr), expr1.Parameters);
    //    }
    //}

    public static class PredicateBuilder
    {
        public static Expression<Func<T, bool>> True<T>() { return f => true; }
        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1,Expression<Func<T, bool>> expr2)
        {
            var parameter = Expression.Parameter(typeof(T));

            var leftVisitor = new ReplaceExpressionVisitor(expr1.Parameters[0], parameter);
            var left = leftVisitor.Visit(expr1.Body);

            var rightVisitor = new ReplaceExpressionVisitor(expr2.Parameters[0], parameter);
            var right = rightVisitor.Visit(expr2.Body);

            return Expression.Lambda<Func<T, bool>>(Expression.AndAlso(left, right), parameter);
        }

        private class ReplaceExpressionVisitor: ExpressionVisitor
        {
            private readonly Expression _oldValue;
            private readonly Expression _newValue;

            public ReplaceExpressionVisitor(Expression oldValue, Expression newValue)
            {
                _oldValue = oldValue;
                _newValue = newValue;
            }

            public override Expression Visit(Expression node)
            {
                if (node == _oldValue)
                    return _newValue;
                return base.Visit(node);
            }
        }
    }
}

